Vacuum circuit interrupter



Aug. 17, 1937. w. K. RANKI VACUUM CIRCUIT INTERRUPTER Filed Feb.' 25, 1954 2 Sheets-Sheet l- TWVSTWJCOTI WiHam K. Hamm. b9 7+ M/2M I-H A tor-heg Aug. 17, 1937. w, K RANKIN A2,090,519

VACUUM CIRCUIT INTERRUPTER Filed Feb. 23. 1934 2 Sheets-Sheet 2 Inventor;

\ William KRahkih, b5 #aA/U15. yu/w1 A torhey.

His

Patented Aug. 17, 1937 UNITED STATES PATENT OFFICE Genera-l Electric Company, a corporation of New York Application February z3, 1934, serial No. 712,521

20 Claims.

My invention relates to vacuum circuit interrupters, more particularly to high vacuum switches for interrupting high tension power circuits, and has for its principal object the provision of an improved vacuum switch of the aforesaid type which shall have greatly increased interrupting capacity and whichshall be simple and compact in construction andeflicient and reliable in operation.

High vacuum circuit breakers capable of consistent interruptions of R. M. S. currents `up Ato 5,000 amperes at 12 to 15 kv. have heretofore been constructed. The operation beyond this 1r point was unpredictable and fortuitous. The real limitations on the interrupting capacity of this type of circuit breaker, however, were not recognized prior to the present invention.

I have found that successful operation of a lhigh vacuum circuit interrupter depends on the 20 amount and pressure of metallic vapor surrounding the coacting electrode or Contact surfaces of the interrupter when the zero point of the alternating current wave is reached. Assuming that the contacts are composed of copper, the

25 amount of copper vapor generated depends on the R. M. S. amperes at the cathode spot and the rapidity of motion of the cathode spot over the contact surface, the maximum generation of copper vapor occurring when the cathode spot is sta- 30 tionary. The theory involving the formation and maintenance of the cathode spot need not be considered in detail for an understanding of the 4present invention, it being sufficient to point out that upon separation of the contacts there is lo- 35 calized heating and emission of electrons at one of the contacts, depending upon the polarity thereof at the time of contact separation, said localized condition or cathode spot existing on that contact until the zero value of the alternat- 40 ing current wave is reached after which it reestablishes on the other contact under favorable conditions. The cathode spot serves to support and maintain flow of current between the contacts by reason of emission of electrons and ionil45 zation of metallic vapor and gas formed by heating, the emission serving to ionize by collision the metallic vapor and gas adjacent the contacts, which results in bombardment of the cathode spot, further heating thereof, and consequent 50 generation of more metallic vapor. 'I'he presence of metallic vapor, therefore, aids the ow of current by reason of the ionization thereof.

On the other hand, the absence of metallic vapor around the contact surfaces at the current Zero 55 greatly reduces` the possibility of re-formation of the cathode spot. Under such conditions high tension power circuits of large amperage may be interrupted in one-half cycle.

The dissipation of this metallic vapor can be effected only through condensation thereof on the surrounding walls. The condensation how- A ever is limited by the fact that the iirst few molecular layers do not instantly dissipate their heat to the walls and the following metallic molecules are consequently reflected.

I have furthermore found that the generation of metallic vapor may be substantially eliminated and the interrupting capacity of the high vacuum switch increased many times ,by moving the cathode spot at high velocity over the contact surfaces and that this movement of the cathode spot and associated electron stream may be effected by means of a magnetic field.

The use of a magnetic field in both air and fluid-break or oil circuit breakers for the purpose of facilitating interruption of the arc is well known practice. Such use of a magnetic field is based on the fact that the resistance of the arc may be increased by mechanically stretching and reducing the cross sectional area of the same through a' dielectric or insulating material or by forcibly moving the arc into intimate engagement with a surrounding insulating medium. By so breaking up and increasing the resistance of the are within an insulating fluid as air or oil,

interruption of the arc is greatly facilitated.`

,However in the case of vacuum switches the use of a magnetic field has heretofore been considered Without value, particularly in view of the characteristic rapid wandering action of the cathode spot in vacuum devices. In other words, the cathode spot, which serves to support and maintain iiow of current between the electrodes or contacts of a vacuum switch, inherently travels over the contact surfaces at random and at varying speed, no special provision being made to obtain this action. Furthermore .the method of breaking up and increasing the resistance of the arc,"which in the case of a vacuum switch is in the nature of an electron discharge, by means of an`insulating fluid in the manner above described is of course eliminated.

Notwithstanding the peculiar characteristics of the vacuum switch which would indicate the use of a magnetic field to be Without value in inter.

rupting a high tension power circuit, I have found that a magnetic field may be utilized adjacent vacuum switch contacts so as not only to improve the interrupting characteristics of the switch but also greatly to increase the interrupting capacity thereof, f

In accordance with my invention the electron stream in the gap between the coacting contact or electrode surfaces is subjected to a magnetic field so as definitely and rapidly to move said stream and the cathode spot about said contact or electrode surfaces at such a rate that substantially no metallic vapor is formed in said gap. More particularly the cathode spot is rotated at very high speed or spun about the contact or electrode surfaces by a radial magnetic field during interruption of the circuit in a preferred form of the invention.

My invention will be more fully set forth in the following description referring to the accom.- panying drawings, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Referring more particularly to the drawings, Fig. 1 is an elevational view, partly in section, of a high vacuum switch embodying the present invention for interrupting high tension power circuits; Fig. 2 is an elevational end View of the vacuum switch shown in Fig. 1; Fig. 3 is an elevati'onal View, partly in section, of another form of high vacuum switch embodying the present invention; Fig. 4 is a plan sectional view of the vacuum switch shown in Fig. 3; Fig. 5 is an elevational view, partly in section of another form of vacuum switch together with operating mechanism therefor embodying the present invention; and Fig. 5a indicates the magnetic field between the contacts of Fig. 5 upon separation thereof.

The vacuum switch illustrated by Figs. 1 and 2 comprises a highly evacuated casing I in which are disposed coacting relatively movable switch contacts 2 and 3. The operating means' for the switch contacts comprises in the present instance a pivoted arm 4 connected as at 5 to suitable motive means and as at 6 within the evacuated casing to contact 3. The operating arm 4 extends through and is sealed to a flexible metallic diaphragm 'I which is in turn sealed as at 8 to the casing I so as to form a wall portion thereof.

The pivotal mounting for the arm 4 comprises a yoke member 9 which is connected to the arm 4 and pivotally mounted at I0 on the exterior of the casing. The pivot IIJ is located so that its center line is substantially in the plane of the diaphragm 7 as indicated in Fig. 1 so as to minimize exing of the diaphragm upon pivotal movement of the operating arm 4.

The stationary contact 2 coacting with the movable contact 3 is connected to and mounted on the lower end of a conductor stud which is mounted in andinsulated with respect to a wall of the casing I in any suitable manner. In the present instance the conductor stud I is insulated from the casing I by a pair of insulating collars or sleeves I2 and I3 coacting with flanges I4 and I5 respectively which are secured at opposite ends of the conductor stud. The sealing means for the conductor stud comprises an insulating sleeve I6 which is sealed by means of ilexible metallic diaphragms I'l and I8 to the casing and the conductor stud respectively. The insulating mounting and sealing means for the conductor stud form no part of the present invention and are described with more particularity and claimed in applicants Patent No. `1,905,751 granted April 25, 1933 for Vacuum sealing structure.

The casing I and the diaphragm 1 'as illustrated are composed of a suitable metal for keeping high vacua, the metal comprising at least the side walls of the casing being non-magnetic. 'I'he contacts 2 and 3 preferably are composed of a low-resistance material as copper which may be readily freed of occluded gases. The switch is mounted on an insulating support comprising insulators 20 on which supporting structure 2| for the switch casing is mounted.

For the purpose of utilizing a magnetic field adjacent the switch contactsZ and 3 in the manner previously described, an electromagnet 22 connected in series with the circuit to be interrupted is provided with pole pieces 23 located at opposite sides of the casing I so that the contacts 2 and 3 vare disposed between the same as illustrated. 'I'he circuit through the switch therefore comprises the terminal and conductor stud II, contacts 2 and 3, flexible conductor 24 interconnecting the contact 3 and terminal 25 sealed through a wall of casing I, conductor 26, electromagnet 22 and terminal 21. The wall portions of the casing I opposite the poles of the electromagnet maybe composed of a high-resistance, non-magnetic metal so that circulating currents in the casing are substantially limited.

Therefore upon separation of the switch contacts in response to counter-clockwise rotation of the operating arm 4, the electron discharge between the contacts for sustaining current flow is subjected to a strong magnetic eld transversely thereof which causes shifting of the electron stream and the cathode spot over the coacting contact surfaces and towards the outer tips of the contacts at a high rate of speed. The shifting of the electron discharge and the cathode spot takes place at very high speeds as in vacuum tubes so that there will be little, if any, metallic vapor boiled from the contacts during the circuit interrupting operation. The operation of the Vacuum 'switch is very fast, the current generally ceasing within one-half cycle of alternating current at commercial frequencies. Furthermore, destructive burning of the contacts is eliminated and an extreme degree of degassing is rendered unnecessary due to the fact that there is no deep burning of the contacts.

A detailed description of the method of constructing the vacuum switch is believed to be unnecessary other than to point out that well known methods of degassing the switch structure and obtaining high vacua maybe used. All current-carrying parts of the switch structure and likewise parts exposed to heat are suitably degassed and the switch casing is evacuated, preferably to as high degree as can practically be obtained.

Figs. 3 and 4 illustrate a form of my invention wherein electromagnetic means are arranged to induce currents in the contact structure, the magnetic field resulting from the interaction of the main magnetic field and that produced by the aforesaid induced currents being in such a direction in the gap between the contact surfaces upon opening of the circuit that the cathode spot is rapidly rotated or spun about said contact surfaces.

Referring morc particularly to the drawings, an evacuated casing 30 has disposed therein relatively mfivable contact structure comprising a stationary disk-like contact member 3|, an annular contact member 32 surrounding and spaced with respect to the contact 3|, and a movable bridging contact member 33 for interconnectingr the contacts 3| and 32. As best illustrated by Fig. 4, the contact 32 comprises a ring split as at Cri vaecomo Gili?, for the purpose of minimizing induced or eddy currents, and the bridging contact member 33 is provided with three contact portions 33 havingl wedge-shaped contact surfaces corre'- sponding to the coacting Contact surfaces of the contacts 3l and 32.

The bridging member 33 is provided with an operatingy rod 34 extending through the switch casing for connection to suitable motive means and is sealed to the casing by means of a r'ienible metallic bellows 35. The operation or the bridging member in opening and closing the circuit between the contacts 3l and 32 is believed to be obvious without further description.

The electromagnetic means for producing a field as above described upon `opening oi the circuit comprises an electromagnet 36 surrounding the contact structure exteriorly of the casing 33. The electromagnet 36 comprises a spirally wound conductor 3l, lthe outer turn extending as at 3S to one of the vacuum switch terminals, and the inner turn electrically connected in a manner presently described to the contact 32. The magnetic circuit of the electromagnet comprises aplurality of U-shaped members 39 composed of suitable magnetic material as iron and arranged radially with respect to the coil 3'l and abutting the switch casing 3D as illustrated by Fig. 4. )insulating spacers 4G coacting with a band 4l serve to maintain the magnetic members 39 in position and to strengthen the structure.

For the purpose of minimizing heating of the switch casing and for directing a strong magnetic eld adjacent the contact structure, the casing 3U is composed of two sections 30 and 3i?" interconnectedy by a band 42 composed of a suitable non-magnetic high-resistance metal. Or

A in lieu of this the entire switch casing may be constructed of such material. The contact 32 f and the inner turn of the coil 31 are electrically connected to the wall portion 42 at opposite sides thereof, preferably at a low resistance point especially provided for such connection. The legs of the magnetic members 39 abutthe outer side of the wall portion 42 as illustrated, theswitch casing being provided with slotted magnetic pole pieces 43 at'the inner side of the wall portion 42 and opposite the leg portions of the electromagnets. It will therefore be apparent 50 that energization of the electromagnet produces a magnetic iield within the switch casing between the coacting pole pieces 43.

During normal operation of the vacuum switch the bridging member 33 is elevated to inter- 55 connect contacts 3| and 32 and the circuit through the vacuum switch includes the insulated lead-in conductor 44 which may constitute one terminal of the switch, contacts 3|, 33 and 32, the nonrnagnetic wall portion 42. and magnet coil 3l. Upon opening the circuit bylowering of the bridging member 33 the annular gap between the contact surfaces of contacts 3l and 32 is traversed by a magnetic field generally as indicated by Fig. 3. The induced currents set up in the contact member 3l by the main magnetic neld of the electromagnet 36 in turn produce a neld in Opposition to the magnetic eld so that the resulting eld traverses the annular gap throughout in a denite direction so as to cause rapid rotation of the electron stream and cathode spot about the contact surfaces. This rotation of thc cathode spot, as previously pointed out, is extremely rapid and increases the interrupting capacity of the vacuum switch many times. As in the previous instance the circuit is generally interrupted within one-half cycle and there is no noticeable burning or the contacts.

Fig. illustrates another form of my invention wherein a. radial field traversing an annular gap between the contacts is produced by electromagnets whose elds oppose each other.. fore vparticularly the vacuum switch comprises casing 1125 within which relatively movable annular contacts i6 and il are disposed. Each of thecontacts is electrically connected to a helical conductor as indicated at and 'l, respectively, the coils formed by said conductors being wound in opposite directions so that the magnetic fields of the respective coils are in opposition to each other. The upper contact and associated coil lid are connected to and supported at the lower end of an insulated lead-in conductor stud d8 forming one terminal of the switch. The lead in conductor stud is mounted and sealed with respect to the casing de generally in the manner described with reference to mg. l. At the lower end or the conductor stud a shield Lligenerally conical in form, is provided for the purpose of preventing short circuiting oi the insulating collar l2 by a deposition of metallic particles.

The movable contact @l and coil ril" are carried by an operating arm til pivotally mounted as at 5l exteriorly of the casing and sealed to the casing by a flexible metallic diaphragm 5i? as in Fig. l. The movable contact structure above described is electrically connected as by a flexible conductor tatto the metallic casing t5 which has formed thereon the other terminal 5d.

Upon opening of the switch contacts de and 4l by counter-clockwise rotation of the operating arm 5d the opposing elds produced by the coils 46 and rtl produce a strong radial eld transversely of the annular gap between the contacts as illustrated by Fig. 5a so that the electron stream and cathode spot are rotated about the contact surfaces atv great speed as in Fig. 3. The coils 4G and dl', which are connected in series with the contacts 36 and 4l, and the circuit to be interrupted therefore produce a powerful magnetic eld which functions positively to shift the cathode spot at very high speed causing very eillcientv and prompt interruption of the circuit.

In View of the fact that the elds oi the coils 46 and 4l.' are in' opposition it will be apparent that the contacts 46 and 4l are normally biased away from each other, the repelling force depending on the current traversing the switch. When the operating arm 50 is maintained in closed circuit position, the resilience of the coils 46' and 4T', which are under compression in the closed circuit position, maintains the contacts in proper engagement.

For the purpose of utilizing the aforesaid repellant force so as to cause opening of theyswitch upon occurrence of a predetermined overload or short circuit, operating and tripping mechanism is provided responsive to such overload or short circuit conditions. To this end the operating arm 50 is operatively connected to the actuating solenoid 55 through resilient means, as a compression spring 56, a thrust-transmitting andtripping toggle 51 related to the switch arm 50, and a centrally pivoted lever 58. In the closed circuit position shown the switch arm 50 is maintained in position by the bias of spring 56 which is in turn held in compressed position by the overset toggle 51 and latch 59 acting at the opposite end of the lever 58. The switch may be tripped through the latch 59 by means of a trip- Gli g in

ping solenoid Bilactuated either by push button control or in response to any abnormal circuit condition. 'I'he tripping in response to predetermined overload is however regulated by the tension of spring 56 which may be adjusted in any suitable manner. When the repellant force at the switch contacts is suiicient to move the switch arm 50 slightly counter-clockwise against the bias of spring 56 the extension 50' of the l switch arm engages the extension 51' of one link of the overset toggle 5l so as to cause buckling of the toggle and opening of the switch by spring 6|. The spring 56 obviously is stronger than spring 6|, the latter spring being merely for the l5 purpose of assisting in the switch opening operation. In the circuit closing operation, the actuv`ating solenoid 55 causes counter-clockwise rotaction of the lever 58 so as to close the switch i through the thrust-transmitting toggle 51 and 20 spring 56. The latch 51 is reset in its thrusttransmitting position by release of the latch 59 which causes clockwise rotation of lever 58. The above described mechanism is trip-free in operation, that is the switch cannot be held closed while 25 an abnormal condition obtains in the circuit.

It should be understood that my invention is not limited to specific details of construction and arrangement thereof herein illustrated, and that changes and modifications may occur to one 30 skilled in the art without departing from the spirit of my invention. g What I claim as new and desire to secure by Letters Patent of the United States is:

1. A vacuum switch for interrupting high ten- 35 sion power circuits, comprising a highly evacuated casing, relatively movable contacts substantially freed of occluded gases disposed within said casing, operating means for said contacts, and means for producing a magnetic eld adjacent l4 0 the arc path formed by separation of said contacts for rapidly shifting the cathode spot on said contacts at such a rate that substantially nometallic vapor is formed in said casing.

2. A vacuum switch for interrupting high ten- 45 sion power circuits, comprising a highly evacuated casing, relatively movable contacts substantially freed of occluded gases disposed Within said casing, operating means for said contacts, and means for producing a magnetic field adjacent the arc 50 path formed by separation of said contacts for rapidly and continually shifting the cathode spot over the contact surfaces of said contacts at such a rate that substantially no metallic vapor is formed in said casing. 55 3. A vacuum switch for interrupting high tension power circuits, comprising a highly evacuated casing, relatively movable contacts substantially freed of occluded gases disposed Within said casing, operating means for said contacts, and 60 means for producing a magnetic field between said contacts upon separation thereof for rapidly rotating the electron stream and cathode spot with respect to the surfaces of said contacts at such a rate that substantially nq metallic vapor 65 is formed in said casing. t* V 4. A vacuum switch for interri ting high tension power circuits, comprising,j af' highly evacuated casing, relatively movable contacts disposed within said casing, operating means for said con- 70 tacts, and means for producing opposing magnetic `fields between said contacts upon separation thereof for causing rapid rotation of the electron stream and cathode spot with respect to the contact surfaces at such a rate that substantially no 75 metallic vapor is formed in said casing.A

`5. A vacuum switch for interrupting high tension power circuits, comprising a highly evacu- Iated casing, relatively movable contacts arranged to form an annular gap between the same uponV separation thereof, operating means for said contacts, and means for producing a radial magnetic eld within said gap so as to cause rapid rotation of the cathode spot over the contact surfaces defining said gap at such a rate that substantially no metallic vapor is formed in said casing.

6. A vacuum switch for interrupting high tension power circuits, comprising a highly evacuated casing, relatively movable contact structure disposed within said casing comprising a centrallypositioned contact member, a ring-like contact member forming with said first-named member an annular gap, and a bridging member for interconnecting said contact members, operating means for effecting relative movement of' the bridging member with respect to said contact members, and an electro-magnet Vsurrounding said casing for producing a magnetic field in said annular gap upon opening of the circuit so as to cause rapid rotation of the electron stream about said contact members.

7. A vacuum switch for interrupting high tension power circuits, comprising a highly evacuated metallic casing, said casing having a wall portion composed of a non-magnetic high resistance metal, relatively movable contact structure disposed within said casing arranged to form an annular gap upon opening of the circuit, said contact structure electrically connected to said wall portion within said casing, operating means for said contact structure, and an electromagnet exteriorly of said casing and adjacent said wall portion for producing a magnetic eld between the coacting contact surfaces of said contact structure, said electromagnet electrically connected to said wall portion exteriorly of said casing.

8. A vacuum switch for interrupting high tension power circuits, comprising a highly evacuated casing, relatively movable contacts disposed within said casing, said contacts electrically con-- nected to helical conductors arranged so that the magnetic field produced thereby during normal operation of the switch tends to bias said contacts apart, operating means for said contacts including a contact operating member, resilient means opposing said magnetic bias, and tripping means interconnecting said member and said resilient means arranged to release said resilient means 4and permit opening of the switch contacts when said magnetic bias exceeds a predetermined magnitude.

9. The combination with an electric switch having contacts normally biased towards open circuit position by the magnetic forces of the circuit to be interrupted, of operating and tripping means for said switch comprising a spring for maintaining-said contacts closed against said magnetic bias, and tripping means operatively connected to said contacts for causing release of said spring and opening of said contacts when said magnetic bias exceeds a predetermined Vmagnitude.

10. The combination with an electric switch comprising relatively movable contacts normally biased towards open circuit position by themagnetic forces of the circuit to-be interrupted, of operating means for said contacts comprising an actuating member, a spring connected to said member for maintaining said contacts in engagement against the bias of said magnetic forces,

motive means, ineens operatively interconnecting said motive means and said spring including a tripping toggle, separate tripping means operatively connected to said interconnecting means,

5 and a spring connected to said actuating member f for biasing said contacts towards open circuit position, said tripping toggle and actuating membeibeing operatively connected so that said firstnamed spring is released when the magnetic bias lo on said contacts exceeds a predetermined inagnitude thereby causing separation oi said contacto.

il.. .d vacuum switch for interrupting high tension power circuits comprising an evacuated caslo ing, relatively movable contactsdisposed within said casing, electromagnetic means connected in series with said contacts arranged to induce currents in one oi said contacts upon opening of the circrdt the resulting magnetic iield adjacent saidcontacts causing rapid rotation of the cathode spot on said contacts at such a rate that substantially no metallic vapor is formed in said casing, and operating means for eecting relative movement of said contacts.

i2. A vacuum switch for interrupting high tension power circuits comprising an evacuated. casing, relatively movable contacts disposed Within said casing arranged to form a substantially annular gap upon separation thereof, electromag- 30 netic means arranged toinduce currents in one of said contacts -upon opening oi the circuit, the resulting magnetic eld Within said annular gap causing rapid and definite rotation oi the cathode spot on the coacting contact surfaces at such a rate that substantially no metallic vapor is formed in said casing, and operating means for eiecting relative movement of said contacts.

13. A vacuum switch for interrupting high tension power circuits comprising an evacuated cas- 40 ing, relatively movable contact structure disposed Within said casing, electromagnetic means connected in series with said contact structure for producing a magnetic field in the gap between the coacting contact surfaces upon separation thereoi, said magnetic eld causing rapid and denite movement of the cathode spot on said contact surfaces at such a rate that substantially no metallic vapor is formed in said gap, and operating means for said contact structure.

14. A vacuum circuit interrupter for high tension, alternating current power circuits comprising a highly evacuated casing, coacting electrodes substantially freed of occluded gases disposed Within said casing, said electrodes being separated 55 by a gap upon opening of the circuit? and means for applying a magnetic eld at said gap for rapidly moving the cathode spot formed on said electrodes at such a rate that substantially no metallic vapor is iormedin said arc gap.

15. In a device of the class described, the oombination of two contacts one of which is movable to engage and disengage the other and is biased for disengaging movement, means additional to the said contacts for normally `resisting the bias 65 of the movable contact and for retaining the said contact in engaging position which retaining means is normally dependent for its effectiveness upon the avoidance of a predetermind change in the pressure between the contacts, and means 70 acting directly on one of the contacts independently ofthe retaining means to effect the said predetermined change in the contact pressure and to thereby cause the retainingmeans to release themovable Vcontact for automatic disengaging 75 movement.

CII

lo. The combination in a circuit breaking device oi the class described, of tvvocontacts one of which is movable to engage and disengage the other and is biased for disengaging movement, means additional to the said contacts for normally resisting the bias or thernovable .contact and :tor retaining the said contact in engaging position which retaining-.means is normally depend- ,ent for its eectiveness upon the contact pressure, and means acting independently of the retaining means to reduce the contact pressure and to thereby cause the retaining means to release the movable Contact for automatic disengaging movement.

l'l. in an automatic circuit breaker, the combination oi two contacts one oi which is movable to engage and disengage the other and is biased for disengaging movement, means additional to the said contacta for normally resisting the bias of the movable contact and `ior retaining the said contact. in engaging position which retaining means is normally dependent for its enfectiveness upon the avoidance of a predetermined change in the pressure between the contacts, and an electroresponsive means operable upon the attainment of a predetermined abnormal current condition in the circuit and acting dire tly on one oi the contacts independently ci the retaining means to efiect the said predetermined change in the contact pressure and to thereby oalise the retaining means to release the movable Contact tor automatic disengaging movement.

18. in an automatic circuit breaker, the combination of two contacts one oi `vfl'iich is movable to engage and disengage the other and is biased for disengaging movement, means additional to the said contacts 'for normally resisting the bias of the movable contact and for retaining the said contact in engaging position which retaining means is normally dependent for its eectiveness upon the avoidance of a predetermined change in the pressure between the contacts, and an electromagnet arranged to be actuated upon the attain- Yment of a predetermined abnormal current condition in the circuit and upon being so actuated serving independently of the retaining means to leffect the said vpredetermined change in the contact pressure and to thereby cause the retaining means to release the movable contact for automatic disengaging movement.

, 19. The combination in an'automatic circuit breaker, of two contacts one of which' is movable to engage and disengage the other and is biased tor disengaging movement, means additional to the said contacts for normally resisting the bias l of the movable contact and for retaining the said contact in engaging position which retaining means is normally dependent for its eiectiveness upon the contact pressure, and an electro-responsive means operable upon the attainment of a. predetermined abnormal current condition in the circuit and acting independently of the reconditions in the circuit and upon being so energized acting independently of the retaining means tov tend to relatively move the substantially sta.- tionary contact so as to reduce the contact pressure and thereby release the retaining means.

WILLIAM K. RANmN. 

